Session J45: Padden Award Symposium

Field-Theoretic Studies of Nanostructured Triblock Polyelectrolyte Gels

Recently, experimentalists have developed nanostructured, reversible gels formed from triblock polyelectrolytes (Hunt et al. 2011, Lemmers et al. 2010, 2011). These gels have fascinating and tunable properties that reflect a heterogeneous morphology with domains on the order of tens of nanometers. The complex coacervate domains, aggregated oppositely charged end-blocks, are embedded in a continuous aqueous matrix and are bridged by uncharged, hydrophilic polymer mid-blocks. We report on simulation studies that employ statistical field theory models of triblock polyelectrolytes, and we explore the equilibrium self-assembly of these remarkable systems. As the charge complexation responsible for the formation of coacervate domains is driven by electrostatic correlations, we have found it necessary to pursue full ``field-theoretic simulations'' of the models, as opposed to the familiar self-consistent field theory approach. Our investigations have focused on morphological trends with mid- and end-block lengths, polymer concentration, salt concentration and charge density.   

Effect of Confinement on Proton Transport in Nanostructured Block Copolymer/Ionic Liquid Membranes

Nanostructured membranes containing structural and proton-conducting domains are of great interest for a wide range of applications requiring high conductivity coupled with high thermal stability. Understanding the effect of nanodomain confinement on proton-conducting properties in such materials is essential for designing new, improved membranes. This relationship has been investigated for a lamellae-forming mixture of poly(styrene-$b$-2-vinylpyridine) (PS-$b$-P2VP) with ionic liquid composed of imidazole and bis(trifluoromethane)sulfonimide (HTFSI), where the ionic liquid selectively resides in the P2VP domains of the block copolymer. Quasi-elastic neutron scattering and NMR diffusion measurements reveal high levels of a fast proton hopping transport mechanism, which we hypothesize is due to changes in the hydrogen bond structure of the ionic liquid under confinement. This, in combination with unique ion aggregation behavior, leads to a lower activation energy for macroscopic ion transport compared to that in a mixture of ionic liquid with P2VP homopolymer. These results portend the rational design of nanostructured membranes having improved mechanical properties and conductivity.   

Controlling Au Nanorod Dispersion in Thin Film Polymer Blends

Dispersion of Au nanorods (Au NRs) in polymer thin films is studied using a combination of experimental and theoretical techniques. Here, we incorporate small volume fractions of polystyrene-functionalized Au NRs ($\phi_{rod} \approx 0.05$) into polystyrene (PS) thin films. By controlling the ratio of the brush length (N) to that of the matrix polymers (P), we can selectively obtain dispersed or aggregated Au NR structures in the PS-Au(N):PS(P) films. A dispersion map of these structures allows one to choose N and P to obtain either uniformly dispersed Au NRs or aggregates of closely packed, side-by-side aligned Au NRs. Furthermore, by blending poly(2,6-dimethyl-p-phenylene oxide) (PPO) into the PS films, we demonstrate that the Au nanorod morphology can be further tuned by reducing depletion-attraction forces and promoting miscibility of the Au NRs. These predictable structures ultimately give rise to tunable optical absorption in the films resulting from surface plasmon resonance coupling between the Au NRs. Finally, self-consistent field theoretic (SCFT) calculations for both the PS-Au(N):PS(P) and PS-Au(N):PS(P):PPO systems provide insight into the PS brush structure, and allow us to interpret morphology and optical property results in terms of wet and dry PS brush states.   

Influence of Charge and Network Inhomogeneities on the Swollen-Collapsed Transition in Polyelectrolyte Nanogels

Polyelectrolyte nanogels are sub-microscopic networks of solvent-permeated polyelectrolyte chains that undergo large reversible volume changes for a range of environmental stimuli. This volume phase transition behavior finds use in targeted drug delivery, optical switching in photonic crystals, and many other applications that require controlled tunability. Although the strength of electrostatic interactions have a strong influence on the nanogel response, these interactions are not well captured by the classical mean-field theories of macroscopic gels. We develop a simplified Poisson-Boltzmann model of spherical gels, that highlights the importance of charge inhomogeneities and the associated Coulomb interactions in determining the response of gels. Our analysis reveals that nanometer-sized gels, collapsed gels, and gels in media with low salinity or high dielectric constant, have large regions of excess charge, and show clear deviations from the classical Donnan picture of polyelectrolyte gels. The detailed swelling-collapse behavior is obtained using the theoretically-informed coarse-grained simulations, which includes the effects of network imperfections and thermal fluctuations. The simulations capture the universal features of volume phase transition in nanogels.   

Enhanced electron mobility of n-channel polymer thin film transistors by use of low-k polymer dielectric buffer layer

Understanding the factors that govern charge transport in polymer thin film transistors is of interest in developing high-performance polymer transistors and circuits. Engineering the dielectric properties of the gate insulator of a field-effect transistor represents one of the promising approaches to improving the performance of the devices. We show that insertion of a low-k polymer dielectric layer between a silicon dioxide gate dielectric and poly(benzobisimidazobenzophenanthroline) (BBL) semiconductor of n-channel organic transistors increases the field-effect electron mobility by two orders of magnitude. The enhanced electron mobility was accompanied by increased on/off current ratio, superior multicycling stability with negligible hysteresis, and enhanced durability in air. Systematic studies of a series of polymer dielectrics showed that the electron mobility increased exponentially with decreasing dielectric constant, which can be understood in terms of the reduced energetic expense of charge carrier/dipole interaction.   

Imaging three dimensional bicontinuous networks in bulk heterojunction solar cells

Highly efficient, solution processable, organic photovoltaics typically consist of a two component donor-acceptor type heterojunction structure comprised of a low bandgap conjugated polymer donor blended with a fullerene acceptor. Efficient charge extraction from these blends demands that donor and acceptor components form nanoscale phase separated percolating pathways to their respective electrodes. Although the existence of this bicontinuous interpenetrating network, termed a bulk heterojunction (BHJ), is hypothesized to be requisite for efficient device operation, attempts to characterize BHJ structures using conventional transmission electron tomography (TEMT) techniques have failed. Energy filtered TEMT (EF-TEMT) is demonstrated to overcome the inadequacies of conventional TEMT, enabling three-dimensional (3D) imaging of high efficiency BHJ structures with nanometer resolution. Considered in combination with x-ray scattering measurements, the 3D chemical maps derived from EF-TEMT are used to offer a plausible mechanism of BHJ formation in devices reaching 7.1{\%} efficiency.   

Single molecule dynamics of self-associating polymers

Recent progress has been made experimentally in understanding the importance of reversible interactions in polymer systems, with systems such as stimuli-responsive gels demonstrating tunable properties. We extend these ideas to single molecules, since this type of behavior is especially important in biological polymers where molecular interactions can be both reversible and long-lived. We have developed simulation models where a Bell-like association model is incorporated into a Brownian Dynamics simulation, so that we represent a self-associating polymer in a coarse-grained fashion. We find that there are exciting implications of these binding properties on the dynamics of polymers both in and out of equilibrium. The introduction of this extra time scale to a dynamic polymer system manifests itself in drastic changes in polymer properties under dynamic loading; in particular, self-associating polymers in shear flow demonstrate large non-monotonic effects and single-molecule pulling scenarios demonstrate pulling behaviors reminiscent of biological molecules. This thus represents a physical model that has intriguing implications in both biological and synthetic polymer systems.